Parasite-insecticide interactions: a case study of Nosema ceranae and fipronil synergy on honeybee

ABSTRACT: In ecosystems, a variety of biological, chemical and physical stressors may act in combination to induce illness in populations of living organisms. While recent surveys reported that parasite-insecticide interactions can synergistically and negatively affect honeybee survival, the importance of sequence in exposure to stressors has hardly received any attention. In this work, Western honeybees (Apis mellifera) were sequentially or simultaneously infected by the microsporidian parasite Nosema ceranae and chronically exposed to a sublethal dose of the insecticide fipronil, respectively chosen as biological and chemical stressors. Interestingly, every combination tested led to a synergistic effect on honeybee survival, with the most significant impacts when stressors were applied at the emergence of honeybees. Our study presents significant outcomes on beekeeping management but also points out the potential risks incurred by any living organism frequently exposed to both pathogens and insecticides in their habitat.

Conclusion of the study:
In conclusion, our findings showed that honeybees co-exposed to the natural stressor N. ceranae and to an environmental concentration of the insecticide fipronil will undergo a significantly higher mortality compared to the sum of the effects induced by each agent acting alone.

Few studies have been done on such interactions in the honeybee and the resulting data illustrate the difficulty to find out the synergy related mechanisms. The economical and ecological value of honeybees renders our results worrying as the scenario of colonies housing both N. ceranae spores and insecticide residues is realistic. Those results also point out the potential risks incurred by any living organism frequently exposed to both pesticides and pathogens in their environment, no matter the sequence of exposure to those agents. Such multiple stressors interactions, endangering honeybees and potentially other communities, deserve additional attention. Finally, understanding the complexity of cumulative risks is a prerequisite for the implementation of more efficient guidelines in the frame of future chemicals regulation.

Commentaires

Dr Jeff Pettis from the US Department of Agriculture was one of the first to put forward the 3-P hypothesis: "We can't just point to any one single factor as being the dominant thing in the decline in honey bee health. Of late, it seems that this has been the dominant issue, that pesticides are driving everything in bee health. "I think there's more of what I call the 3-P principle – poor nutrition, pesticides and pathogens. Those three things are interacting greatly. Nutrition is the foundation of good bee health, and certainly there's some pesticide exposure going on, but it varies widely over time and space. And the pathogens in my opinion are often acting secondarily. But it's the interaction of these three [that matters]. You get three of them lined up and surely you'll have bees in poor health. Even the combination of any two could be problematic."http://www.independent.co.uk/environment/nature/study-reveals-how-bees-r...

There is also a hypothesis that the known adverse effect of neonicotinoid pesticides on grooming and alloo grooming in insects explains why varroa hits harder since the large scale introduction of neonicotinoid pesticides around 2004 (Between 1992 and 2004 neonicotinoids were used small scale. Further accellerated by the EPA ban on diazinon in 2005 they became the most widely used and fastest growing class of insecticides world wide with a present-day global market share of more than 25%.)http://www.bijensterfte.nl/en/node/447